This invention relates to the assembly of wax motors of the type used in household appliances such as washing machines and dishwashers.
Wax motors are disclosed in U.S. Pat. No. 5,572,869 assigned to the assignee of the present invention and hereby incorporated by reference.
In such wax motors, a metal housing holds wax and a piston so that expansion of the wax when the housing is heated, drives the piston outward. The housing may be heated by a positive temperature coefficient (PTC) resistor attached to the housing. The PTC resistor may have one face in both thermal and electrical contact with the metal housing so that the metal housing may both receive heat from the heated element and serve as one electrode for that resistor.
In order to ensure good thermal contact between the housing and the PTC resistor, and to increase the robustness of the assembly, an adhesive may be placed between the housing and resistor. This adhesive may be a thin layer of thermally conductive adhesive that is nominally electrically insulating, but perforated by the operating voltage of 120 volts to allow conduction between the housing and PTC resistor. Alternatively, the adhesive may be a thermally conductive adhesive with electrically conductive particles dispersed therein.
The use of a thin layer of electrically insulating adhesive that is perforated by the line voltage does not always establish a reliable electrical contact between the housing and resistor resulting in an unnecessarily high rejection rate for tested units. On the other hand, electrically conductive adhesives have proven difficult to apply with a propensity to create short circuit paths across the sides of the resistor when the housing and resistor are pressed together and adhesive is squeezed out from between them.
The selection of possible adhesives is limited by the fact that the adhesive must provide some gap filling qualities and the ability to accommodate different coefficients of expansion of the metal housing of the wax motor and the resistor element over a wide temperature range of heating and cooling. It is difficult to find an electrically conductive adhesive that provides the necessary space filling characteristics, compliance to permit thermal expansion, thermal conductivity, and adhesive strength.